1. Field of the Invention
The present invention relates to a method for detecting a size of an original document to be read in an image forming apparatus, such as an image scanner, a copier, a facsimile, etc., which includes an original document reading function. The present invention also relates to an original document reading apparatus and an image forming apparatus capable of detecting a size of an original document.
2. Discussion of the Background Art
It is known that a background original document reading apparatus, such as an image scanner, and an image forming apparatus, such as a copier, a facsimile, a printer, etc., having an original document reading apparatus, generally automatically detects a size of an original document placed on an original document transparent table such as a contact glass. Specifically, as illustrated in FIGS. 13 and 14, a plurality of reflection type sensors 102 are arranged at positions 106-1, 106-2, 106-3, and 106-4 to detect various sizes of original documents, such as B5, A4, B4 (JIS). Such a reflection type sensor 102 is formed from a light emitting element such as a light emitting diode (LED) and a photo acceptance unit such as a photodiode. The reflection type sensor 102 emits light from the light emitting element onto an original document 105 that is set on an original document table 101 aligned with a pair of original document reference positions 103 and 104, which are disposed at left and rear side ends thereof. The photo acceptance unit of the sensor 102 detects and recognizes a presence of light reflected from the surface of the original document. A size of the original document is detected in accordance with a combination of the presences, i.e., the detections, of the reflection light as shown in the Table in FIG. 15.
However, such a method poses challenges that a size and a setting position of an original document below a standard may not be detected, or cost may rise to precisely detect all document sizes, because the reflection sensors are required to be arranged at prescribed positions below the original document table.
As one approach, Japanese Patent Application Laid Open No. 2-207237 proposes that a width and a length of an original document are detected at high resolution by exposing an original document, imaging reflection light from the original document on a line sensor, such as a CCD, and detecting a pixel that reacts in the line sensor.
However, according to such an approach, a light source significantly generates heat and wastes electric power. That is, an illumination lamp, such as a halogen lamp, a fluorescent light, etc., is needed to be continuously turned on when the sizes are detected.
Further, Japanese Patent Application Laid Open No. 9-73141 proposes another type of an original document size detecting apparatus. Specifically, an illumination device is provided to obliquely emit a segment state illumination light to an original document. A photo acceptance unit is arranged perpendicular to an original document table with its light receiving surface arranged in parallel to the segment state illumination light. The photo acceptance unit is formed from a plurality of photo acceptance elements in an array state with their light receiving surface arranged in parallel to a light emission surface of the illumination device. The width of the light receiving surface is designed to be larger as it approaches the illumination device. According to such an original document size detecting apparatus, an original document can be constantly detected finely, even if the original document table vibrates in height, because the segment state illumination section and the surface of the photo acceptance unit are axisymmetric and magnifications at back and forth sides of the photo acceptance element are substantially the same.
However, when a width and a length of an original document are detected, at least two pairs of original document size detecting apparatuses, each of which includes a pair of illumination devices, which are formed from a light emitting element and a lens or a slit, and an array state photo acceptance element, need to be separately positioned beneath the original document table. As a result, the image forming apparatus becomes costly similar to that employing the reflection type sensor.
Further, brightness of a light emitting diode LED is recently improved, and accordingly a LED array is formed from LED elements in a line state and serves as a long size illumination for an original document reading apparatus.
However, various sizes of original documents cannot be detected at high resolution and precision with such a LED array. Further, such a document size detecting apparatus is undesirably costly and wastes electric power.